A fluorescent substance comprising silicate, phosphate, aluminate or sulfide as a host material and containing a transition metal or a rare earth metal as a luminescent center is widely known.
Such fluorescent substances have attracted attention as a phosphor for LEDs emitting a visible light radiation such as white light and their development has moved on. An LED has its phosphor excited by a source of excitation high in energy such as ultraviolet or blue light to emit a visible light radiation.
These conventional fluorescent substances have the problem, however, that as a result of being exposed to the excitation source, they have had their brightness dropped, and a fluorescent substance that is less in brightness drop is being sought.
Accordingly, attention has recently been riveted to nitride or oxy-nitride fluorescent substances as a material that is stable in crystallographic structure and that allows shifting the excitation or emission light towards the longer wavelength side, among which Sialon fluorescent substances have focuses the spotlight of attention.
An α type Sialon doped with Eu ions can be excited by a wavelength in a broad range of ultraviolet to blue light to emit yellow light of 550 to 600 nm, and its utilization as a phosphor useful for a white LED in combination with a blue LED is being investigated (see Patent Reference 1).
In Sialon compositions, there is also a β type Sialon in which Si and N positions in β type silicon nitride are substituted with Al and O dissolving in solid solution, respectively. Patent Reference 2 discloses a fluorescent substances having rare-earth elements added to a β type Sialon and shows that those doped with Tb, Yb and Ag become green light phosphors of 525 nm to 545 nm. Patent Reference 3 discloses a red light phosphor having a base crystal of CaSiAlN3. Nonpatent Reference 1 reports a red light phosphor having Ca2Si5N8 as its base crystal.